why does the author give the christian warriors exaggerated scenes but not the muslim warriors?

Your question really articulates one of the major themes in the book. To put it simply, the Christians are good (deserving all kinds of detail) and the muslims are bad (undeserving of detail). The Song of Roland gives us Good vs. Evil, pure and simple, Star Wars style. The horror of war is not intensified by ambiguous moral justifications, as in Homer's Iliad, nor are heroes deterred by compassion for the enemy, as in the Mahabharata. War is great, even glamorous. The cost is heavy, but only for the heroes. Villains deserve neither compassion nor grief. The Franks represent pure Good; they are moved by the will of God. The Saracens are evil, and on dying their souls are dragged down to hell by devils. Just like the Crusades, the war in The Song of Roland is seen as a holy mission.